Abstract
Pinned and mobile ferroelastic domain walls are detected in response to mechanical stress in a Mn3+ complex with two-step thermal switching between the spin triplet and spin quintet forms. Single-crystal X-ray diffraction and resonant ultrasound spectroscopy on [MnIII(3,5-diCl-sal2(323))]BPh4 reveal three distinct symmetry-breaking phase transitions in the polar space group series Cc → Pc → P1 → P1(1/2). The transition mechanisms involve coupling between structural and spin state order parameters, and the three transitions are Landau tricritical, first order, and first order, respectively. The two first-order phase transitions also show changes in magnetic properties and spin state ordering in the Jahn–Teller-active Mn3+ complex. On the basis of the change in symmetry from that of the parent structure, Cc, the triclinic phases are also ferroelastic, which has been confirmed by resonant ultrasound spectroscopy. Measurements of magnetoelectric coupling revealed significant changes in electric polarization at both the Pc → P1 and P1 → P1(1/2) transitions, with opposite signs. All these phases are polar, while P1 is also chiral. Remanent electric polarization was detected when applying a pulsed magnetic field of 60 T in the P1→ P1(1/2) region of bistability at 90 K. Thus, we showcase here a rare example of multifunctionality in a spin crossover material where the strain and polarization tensors and structural and spin state order parameters are strongly coupled.
Highlights
Domain wall engineering[1,2] in ferroic materials constitutes one of the most promising areas for new applications in nanoelectronics[3] and nanomagnetism.[4]
The potential for dynamic circuitry in ferroelectrics, where domain walls have lower velocities and are typically more narrow, is only starting to be realized.[14−16] Memory devices based on the wall properties of ferroelectrics have the potential for higher density storage that will outperform the bulk material, as the higher number of narrow domain walls means they constitute around 1% of the material by total volume.[17,18]
Phase transitions within a domain wall, as opposed to those in the bulk material, may confer new functionality. This is typified in some nonpolar ferroelastic perovskite oxides such as SrTiO3, where thermal-induced polarity appears at cryogenic temperatures, indicative of a two-dimensional phase transition within the ferroelastic twin walls that are atomistically thin.[19−22] The emergence of new order parameters during a move through a phase transition can lead to “domains with domains and walls within walls”,19 and materials with such multicomponent order parameters should offer both novel functionality and associated switching opportunities
Summary
Domain wall engineering[1,2] in ferroic materials (ferromagnets, ferroelectrics, ferroelastics) constitutes one of the most promising areas for new applications in nanoelectronics[3] and nanomagnetism.[4]. Examples of novel properties include conductivity[5−7] or superconductivity[8] in the domain walls of insulating ferroelectrics, ferromagnetic ordering of ferroelectric antiferromagnets,[3] and unexpected photovoltaic effects concentrated in the domain walls of ferroelectrics where the voltage is higher than the band gap of the parent material.[9−11] The mobility of domain walls allows them to be moved by electric or magnetic fields or by stress with velocities dependent on the wall dimensions and scale of the driving field. This is typified in some nonpolar ferroelastic perovskite oxides such as SrTiO3, where thermal-induced polarity appears at cryogenic temperatures, indicative of a two-dimensional phase transition within the ferroelastic twin walls that are atomistically thin.[19−22] The emergence of new order parameters during a move through a phase transition can lead to “domains with domains and walls within walls”,19 and materials with such multicomponent order parameters should offer both novel functionality and associated switching opportunities
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